The present invention relates to communications systems and methods, and more particularly, to communications systems including devices, such as wireless radio heads, base stations or other transceiver apparatus, connected in cascade by respective communications links, such as T1 links.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular wireless communications systems, such as those designated AMPS (Advanced Mobile Phone System), NMT(Nordic Mobile Telephone)-450 and NMT-900, have long been deployed successfully throughout the world. Digital cellular wireless communications systems such as those conforming to the North American standard IS-54 and the European standard GSM (Global Systems for Mobile Communications) have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as TIA/EIA-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a typical terrestrial cellular wireless communication system 20. The cellular wireless communications system 20 may include one or more terminals 22, such as mobile terminals, radiotelephones or similar devices, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular system may include hundreds of cells, may include more than one MTSO, and may serve thousands of terminals.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between terminals 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Each cell 24 typically has allocated to it one or more dedicated control channels and one or more traffic channels. A control channel is a dedicated channel used for transmitting cell identification and paging information. The traffic channels carry the voice and data information. Through the cellular system 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of the base station 26 is to handle radio communication between a cell 24 and mobile terminals 22. In this capacity, the base station 26 functions as a relay station for data and voice signals.
As illustrated in FIG. 2, a conventional indoor wireless network 200 that communicates with one or more mobile terminals 22xe2x80x2 includes one or more radio heads 210, a Control Part (COP) 220 (sometimes referred to as a Control and Radio Interface;(CRI) or Radio Control Interface (RCI)), and a mobile switching center (MSC) 230. The radio heads 210 may include one or more radio transceivers and are typically distributed around a building or corporate campus, and provide air interface (radio coverage) functions for cells 240 in under control of the COP 220 in a manner similar to the base stations 26 illustrated in FIG. 1. The air interface implemented by the radio heads 210 and COP 220 may take many forms, including, but not limited to, time division multiple access (TDMA) (e.g., per GSM, IS-136 or similar standards), code division multiple access (CDMA) (e.g., per IS-95, CDMA2000, or similar standards), and more traditional frequency division multiple access (FDMA). Although FIG. 2 illustrates two cells 240 served by the respective radio heads 210, a typical indoor wireless network may have several cells, and each cell may be serviced by one or more radio heads.
The radio heads 210 are connected to the COP 220 by communications links 215 over which data (e.g., frames for wireless communications) and control information are conveyed. In an exemplary RBS 884 Pico Cellular Base Station produced by Ericsson, Inc., the assignee of the present application, the links 215 are T1 (also referred to as DS-1) links which convey messages using a proprietary protocol which includes elements of the Link Access Protocol D (LAPD) Layer 2 protocol used on channels under the Integrated Digital Services Network (ISDN) suite of protocols.
As is well known to those skilled in the art, ISDN provides services that offer xe2x80x9cBxe2x80x9d channels that typically carry user data, and xe2x80x9cDxe2x80x9d channels that typically carry control and signaling information (with some user data transmission under certain circumstances). ISDN Basic Rate Interface (BRI) includes two B channels and one D channel, and its physical layer is specified in ITU-T I.430. ISDN Primary Rate Interface (PRI), typically transmitted over T1 links, includes 23 B channels and one channel in North America, and its physical layer is specified in ITU-T I.431. The channel signaling protocol includes Layers 1-3, which follow the Open System Interconnect (OSI) model. The Physical Layer (Layer 1) protocol is specified in ITU-T I.431. The Data Link Layer (Layer 2) protocol is referred to as LAPD, as specified in the Q.921 Recommendations. The Network Layer (Layer 3) protocol is specified in the ITU Q.931 Recommendations.
On a typical T1 link such as one the links 215 of the indoor wireless system 200 of FIG. 2, information is typically transmitted at 1544 kilobits per second (kb/s), in 193 bit Layer 2 frames that occur every 125 microseconds (xcexcsec). A frame includes a 192-bit payload preceded by a framing (F) bit. An Extended SuperFrame (ESF) includes 24 consecutive frames, with the F bits being used to provide framing functions, a block error check (CRC) channel and a data link (DL). The ESF DL may be used for transmission of scheduled (periodic) maintenance messages and unscheduled priority and control codewords related to maintenance of transmission quality on the T1 link.
A structure for a Performance Report Message (PRM) transmitted on an ESF DL is provided in Table 1. A PRM includes a 13-byte information block bracketed by opening and closing flag bytes. The information bytes, including a header and a footer, are structured as an Unnumbered Information frame according to the LAPD protocol. The data content (body) of the frame is a concatenation of four 2-byte. representations of signal performance for respective one second periods. Each PRM typically includes data for the four most recent seconds, which can provide redundancy if the PRM is corrupted in transmission. A PRM is typically transmitted once every second, and is transmitted in 30 milliseconds (msec).
The fields of the PRM may be described as follows:
Although radio heads 210 of the wireless system 200 of FIG. 2 may be separately connected to COP 220 by respective links in a so-called xe2x80x9chub and spokexe2x80x9d configuration, it is often advantageous to connect radio heads 2101, 2102. . . , 210n of wireless communications system 200xe2x80x2 to a COP 220 in a cascade or xe2x80x9cdaisy chainxe2x80x9d fashion, via uplinks 2151, 2152. . . , 215nxe2x88x921 as shown in FIG. 3. For example, in such a cascade configuration, an xe2x80x9cuplinkxe2x80x9d message, e.g., a PRM, originating from radio head 2102 and destined for the COP 220 is first transmitted to radio head 2101 over uplink 2152, and then transmitted from radio head 2101 to the COP 220 over uplink 2151. xe2x80x9cDownlinkxe2x80x9d messages, i.e., messages send away from the COP 220, may be conveyed in a similar fashion.
Cascading can be advantageous, as separate connections from each device to the central unit may be eliminated, thus potentially simplifying the wiring of the network and reducing transmission distances for signals. Cascading can also simplify the addition of devices to the network. In addition, if xe2x80x9cleased linesxe2x80x9d are used, cost of communications in the network may be reduced as the number of leased lines from the central unit may be reduced. For example, cascaded connections may be particularly advantageous in a system in which the COP 220 is located a significant distance from the radio heads 2101, 2102. . . , 210n, e.g., in a separate building or campus. In such a case, a single leased line can be used to connect the COP 220 to one of the radio heads, and the remaining radio heads can be connected in cascade to this one radio head to avoid the need for additional leased lines.
A potential difficulty with using a cascade network configuration lies, however, in identifying which device and/or link to which a message pertains, e.g., in identifying which of the radio heads 2101, 2102. . . , 210n, originally generated a PRM eventually conveyed to the COP 220, or to which communication link it pertains. Although the Q.921 protocol provides the Terminal Endpoint Identifier (TEI) field that can be used to identify a network device, conventional techniques for using this field may suffer from some disadvantages. For example, existing protocols for T1 links used in the aforementioned conventional wireless systems typically require that the TEI field for a PRM be set to a value of zero. The Q.921 Recommendations describe TEI management procedures in which a device is preassigned a TEI value (for TEI values 0-63), or in which a device is dynamically assigned TEI value (64-124) by a message sent by a central unit in response to a request from the device. However, preassigning TEI values to devices may complicate replacement of devices and reconfiguration of the network. Dynamic assignment can obviate some configuration problems, but can increase network traffic due to the negotiation needed to assign TEI values. Accordingly, there is a need for improved techniques for identifying devices and/or messages in a cascaded network configuration.
The present invention may meet this and other needs by revising Terminal Endpoint Identifiers fields of messages as they are passed along a cascade of devices. According to one embodiment of the present invention, improved differentiation of Performance Report Messages (PRMs) transmitted among cascade devices can be provided by receiving a transmitted PRM at a device, such as a radio head of an indoor wireless communications system, revising the received PRM such that its Terminal Endpoint Identifier (TEI) field is revised according to a predetermined transformation, for example, incremented or decremented, and transmitting the revised PRM on to another device of the cascaded devices. The revised PRM may then be received at the other device, and its revised TEI field used to determine the communications link to which the PRM pertains. The TEI is preferably used in conjunction with a value in the Command/Response (C/R) field of the revised PRM, which indicates whether the PRM pertains to a downlink or uplink. These reference techniques can be generalized to other messages that include TEI and/or C/R fields.
In particular, according to one embodiment of the present invention, a Performance Report Message is received from a first device at a second device. The received Performance Report Message is revised such that a value of a Terminal Endpoint Identifier field in the Performance Report Message is revised according to a predetermined transformation, e.g., by incrementing the value in the Terminal Endpoint Identifier field. The revised Performance Report Message is then transmitted from the second device to a third device. The first, second and third devices may comprise devices of a wireless communications network, such as radio heads and/or a Control Part (COP). The received Performance Report Message is preferably received over a first T1 link connecting the first and second devices, and the revised Performance Report Message is preferably transmitted over a second T1 link connecting the second and third devices. According to another aspect of the present invention, the revised Performance Report Message is received at the third device, at which the revised value of the Terminal Endpoint Identifier field of the received revised Performance Report Message is recovered and a communications link to which the revised Performance Report Message pertains is identified based on the recovered revised value of the Terminal Endpoint Identifier field of the received revised Performance Report Message, preferably in conjunction with a value in a Conmmand/Response field in the received revised Performance Report Message.
According to another aspect of the present invention, devices connected in cascade by respective Integrated Services Digital Network (ISDN) control (D) channels communicate by receiving a message from a first device at a second device. The received message is revised such that a value of a Terminal Endpoint Identifier field in the received message is revised according to a predetermined transformation, e.g., by incrementing the Terminal Endpoint Identifier value. The revised message is then transmitted from the second device to a third device.
According to yet another aspect of the present invention, a communications system includes a first device that transmits a Performance Report Message. A second device, coupled to the first device, receives the transmitted Performance Report Message, revises the received Performance Report Message such that a value of a Terminal Endpoint Identifier field in the Performance Report Message is revised according to a predetermined transformation, and transmits the revised Performance Report Message. A third device, coupled to the second device, receives the transmitted revised Performance Report Message, recovers the revised value of the Terminal Endpoint Identifier field of the received revised Performance Report Message, and identifies a communications link to which the revised Performance Report Message pertains based on the recovered revised value of the Terminal Endpoint Identifier field of the received revised Performance Report Message.
In another embodiment according to the present invention, an Integrated Services Digital Network (ISDN) communications system includes a first device that transmits a message on a first channel. A second device receives the transmitted message on the first channel, revises the received message such that a value of a Terminal Endpoint Identifier field in the received message is revised according to a predetermined transformation; and transmits the revised message on a second channel. A third device receives the transmitted revised message on the second channel, recovers the revised value of the Terminal Endpoint Identifier field of the received revised message, and identifies the revised message as pertaining to the first device based on the recovered revised value of the Terminal Endpoint Identifier field of the received revised message.
In another embodiment according to the present invention, a communications device includes a first communications interface circuit that receives a Performance Report Message on a first communications link. A message processing circuit, operatively associated with the first communications interface circuit, revises the received Performance Report Message such that a value of a Terminal Endpoint Identifier field in the Performance Report Message is revised according to a predetermined transformation. A second communications interface circuit, operatively associated with the message processing circuit, transmits the revised Performance Report Message on a second communications link.
The message processing circuit may also be operative to generate status information relating to at least one of the first and second communications links and to generate a second Performance Report message including the generated status information, and may transmit the second Performance Report Message on the second communications link via the second communications interface circuit.
In yet another embodiment of the present invention, a communications device includes a first communications interface circuit that receives a message on a first communications link. A message processing circuit, operatively associated with the first communications interface circuit, revises the received message such that a value of a Terminal Endpoint Identifier field in the received message is revised according to a predetermined transformation. A second communications interface circuit, operatively associated with the message processing circuit, transmits the revised message on a second communication link.